1. Field of the Invention
The invention relates to an apparatus for detecting defects in patterns, particularly defects in chip patterns of photomasks for use in manufacturing semiconductor integrated circuits.
2. Description of the Prior Art
In processes of manufacturing integrated circuits there is a process for photoetching a silicon wafer. In this process a mask having a desired pattern is placed on a photo-lacquer layer applied on the silicon wafer and the photo-lacquer layer is irradiated by visible light or an ultraviolet ray through the mask. Then the silicon wafer is selectively photoetched in accordance with the mask pattern. The defects in the mask having the pattern printed thereon might affect the yield of the manufactured integrated circuits. The mask is formed by depositing a metal film such as chromium on a glass plate having a sufficiently flattened surface and then by printing a desired pattern on the surface. If there are pin holes in the metal film, the printed pattern might have defects. The present inventors have developed an apparatus for detecting automatically such pin holes in the metal film of the mask pattern with high accuracy.
The photomask has various defects in its pattern as well as the pin holes. The defect detecting apparatus according to the present invention is particularly suitable to detect such defects in the printed pattern of the photomask.
FIG. 1 shows schematically a photomask 1 which is used for manufacturing semiconductor integrated circuits. In the mask 1 there are formed a number of identical chip patterns 3 which are divided by a number of orthogonal scribe lines 2.
FIG. 2 is a microscopic image of a part of the chip pattern 3. This part of the pattern has no defect and thus is a perfect one. The pattern is composed of transparent portions 4 and opaque portions 5. FIG. 3 is also a microscopic image of the corresponding part of another pattern which includes various defects. Portions A and B are residual parts of the metal film. At the portion A the residual part bridges two adjacent lands which should be separated from each other. Thus this residual portion A should be detected as a real or true defect. The other residual portion B exists in a space and in most cases this portion B might not injure the integrated circuits. At a portion C a part of a land is lacking. However, this land is not completely separated and thus this portion C might not affect the integrated circuits. At a portion D a land is completely cut away and this causes a serious adverse influence on the integrated circuits.
Up to now there have been developed the following methods have been developed for detecting the above mentioned defects in the mask pattern.
(1) The mask is inspected by means of a microscope so as to find the defects. In general the pattern is formed by straight lines which intersect perpendicularly with each other, while most defects have irregular shapes as shown in FIG. 3. Therefore the defects can be found in a relatively easy manner. However, this method requires a lot of time and labor and thus is not suitable for detecting the defects in the photomask used in manufacturing the integrated circuits which have a number of chip patterns.
(2) As shown in FIG. 4 a sample mask 7 which has a perfect pattern is prepared and images of this sample mask 7 and the mask 6 to be tested are inspected in a superimposed manner. In this case the image of the mask 6 to be tested is colored in red and the image of the sample mask 7 is colored in green which is complementary to red. For this purpose there is arranged a red color light source 9 and the mask 6 to be tested is irradiated by red light emitted from the source 9. The red light passing through the mask 6 is made incident on an inspection eye 14 by means of an objective 10, a mirror 11, a half mirror 12 and an eye piece 13. The sample mask 7 is illuminated by a green light source 15 and the green light passing through the sample mask 7 is made incident upon the inspection eye 14 by means of an objective 16, a mirror 17, a half mirror 18 and the eye piece 13. When the sample mask 7 having no defect as shown in FIG. 2 and the test mask 6 having the defects as illustrated in FIG. 3 are inspected in a superimposed manner, the portions A and B are seen in green, because in these portions only the green light from the sample mask 7 reaches the inspection eye 14. The portions C and D are seen in red, because in these portions only the red light from the mask 6 reaches the eye 14. The transparent portion other than the portions A, B, C and D can be seen in white, because in the transparent portion both the green and red light rays from the masks 6 and 7, respectively reach simultaneously the inspection eye 14. The opaque portion 5 is seen, of course in black. The defect portions are seen in green or red and the portions having no defect are seen in black or white. Thus the defects can be found in a simpler manner. The masks used in manufacturing the integrated circuits have formed therein a number of identical chip patterns and in order to check such a mask it is necessary to arrange the mask 6 to be tested and the sample mask 7 on a same carrier stage 19 and to move this carrier stage 19 slightly so as to check the successive chip patterns. In case of inspecting the two images of the masks 7 and 6 in the superimposed manner two images must be aligned accurately. If there is an error in this alignment it is impossible to detect the defects accurately. In particular when the two masks 6 and 7 are placed on the same table 19, the masks must be aligned with X and Y directions of the movement of the table. If there is an error in this alignment, the error in superimposition of the two images will increase in accordance with the movement of the table 19. A play in the carrier table 19 also affects the superposition of the two images. Morever since this method is effected with the naked eye, the inspector might be tired causing human errors which cannot be avoided. Also, a long time period is required for inspection.
(3) Electric signals corresponding to a sample pattern which does not include a defect have been previously stored in a record medium such as a magnetic tape or memory elements using an electronic computer. The image of the mask to be tested is picked up by means of a microscopic television camera to produce a video signal. This video signal is compared with the previously stored signal of the sample pattern so as to detect the defects in the checked mask. This method has an advantage in that the defects can be detected automatically without using a visual inspection. However an apparatus for carrying out such a method is liable to be very large and complicated in construction and thus the apparatus becomes quite expensive.
In order to avoid the disadvantages mentioned above the inventors have designed an apparatus comprising a single camera tube on which images of identical portions of two patterns to be checked are focussed in a superimposed manner and defects in the patterns are detected by detecting an amplitude of the output video signal from the camera tube. In this apparatus the defects are represented as gray tones in the video signal and the gray tones are detected by means of an amplitude limiter. However the accuracy of the defect detection was low, because the fluctuation of the amplitude of the video signal is large. In order to obviate this disadvantage the inventors have further developed a method in which use is made of two camera tubes on each of which a respective image of the two patterns is formed and defects in the patterns are detected by comparing two output video signals from the two camera tubes. In this method the accuracy of the defect detection is a greatly improved as compared with the method in which only the single camera tube is used. However it has been found that it is quite difficult to make two camera tubes operate identically. Moreover in case of using the camera tube the carrier table on which the masks to be compared are placed must be transported intermittently due to the residual image effect of the camera tube. This results in a very complicated driving mechanism for the carrier table. The operation speed of the camera tube is rather slow and a time period of 70 to 100 ms is required for checking each field of view. Therefore a quite long time is required for detecting the defects in a number of patterns of the mask.
The inventors have further devised a checking apparatus which can avoid all of the above mentioned drawbacks and check the defects in patterns accurately and speedily with a simple construction. This apparatus comprises means for producing a scanning light spot, an optical system for directing the spot onto two identical portions of patterns to be compared with each other, a pair of light receiving devices each receiving a light transmitted through or reflected from respective pattern portions, circuit means for inverting a phase of an output signal supplied from one of said two light receiving devices and circuit means for mixing the phase inverted output signal and a non-inverted output signal from the other light receiving device. According to such an apparatus the adjacent patterns 3 formed in the photomask 1 shown in FIG. 1 for use in manufacturing the integrated circuits can be compared with each other and defects can be detected with high accuracy. After various experiments it has been further found that the pattern might be detected as having defects even when the relative position of two patterns to be compared with each other deviates slightly and/or these patterns differ from each other only slightly. This results in that masks are unnecessarily rejected as defective masks. That is to say the slight deviation of patterns and/or slight differences in patterns are detected as defects even if they do not affect the manufacture of integrated circuits and thus such masks should not be identified as defective.
The reasons for producing the above mentioned pseudo-defects may be summarized as follows:
(1) A pair of lens systems for forming two images of two patterns to be compared have different distortion characteristics.
(2) A stage for carrying the mask having the patterns to be compared rotates slightly during its travel causing the two images to deviate slightly with respect to each other.
(3) A distance or pitch between the successive chips has an error (about 0.5 .mu.m) due to inaccuracy of a repeater used in manufacturing the photomasks.
(4) Contours of the pattern images fluctuate due to noise in the picture signals.
(5) If the glass plate of the photomask has less plainness, the two lens systems cannot be simultaneously focussed correctly.
Among the above mentioned causes the first and third ones are important or serious. But the problem with respect to the repeater has been improved because nowadays fine patterns have been required and the deviation in chip pitch can be made very small.
The inventors have further devised an improved checking apparatus which can effectively remove the pseudo-defects which should not be detected as true defects. This apparatus is disclosed in U.S. patent application Ser. No. 746,584 filed on Dec. 1, 1976 now U.S. Pat. No. 4,123,170
This apparatus comprises a pattern scanning device for scanning optically identical portions of two patterns to be compared with each other to produce corresponding picture signals, circuit means for subtracting one of the picture signal from the other picture signal to produce a difference signal denoting pattern differences, delay means for delaying the difference signal and circuit means for receiving said delayed difference signal and the non-delayed difference signal to produce an output defect signal in which any pseudo-defects having dimensions smaller than a given dimension determined by the delay time have been removed.
According to the last mentioned defect detecting apparatus as far as a vicinity of the contour of pattern is concerned patterns are compared with each other after their contours have been thinned by a predetermined length and thus the slight registration error and the small defects at the contours can be ignored.
It has been found that when the defects in the patterns are detected by means of such an apparatus in which the pseudo-defects in the vicinity of the pattern contours are removed, it is necessary to increase the width of the contour regions if the two lens systems of the scanning means have differences in characteristics such as distortion and magnification, and thus the defect detection sensitivity is naturally decreased. In other words it is impossible to utilize the maximum resolution of the whole apparatus including the optical system and electric circuitry and the effective resolution becomes very low. In order to decrease the distortion of the lens system it is necessary to use very complicated and expensive lens systems. It should be noted that the scanning means comprise a pair of lens system which simultaneously project the scanning light spots onto the identical pattern portions, the differences in distortion and/or magnification between the two lens systems result in the registration error of the two scanned pattern portions, which error might be detected as a defect.